1. Field of the Invention
The invention relates to a multi-voltage power supply system, and more specifically to a dual-voltage power supply system for vehicles having a two-voltage storage system such as a 14/42 V two-voltage system.
2. Background Art
Multi-voltage level power supply systems, such as 14/42 V two-voltage (“dual voltage”) systems, are known in the art. In the case of 14/42 V systems, such systems have been developed in order to facilitate the transition from a conventional 14 V electrical system to an electrical system operating at a 42 V level. However, one disadvantage of these systems is the need for more than one energy storage device in order to accommodate the energy requirements for high power output loads.
In a conventional automobile, for a example, a single 12 V battery is used as the device for energy storage for the 14V bus. However, in many applications for 14V/42V systems, it is often necessary to use two separate devices for energy storage in order to allow arrangements with the characteristics of the two batteries. Against this background, it is desirable to provide a vehicular power supply system having a two-voltage storage system, wherein costs can be reduced, weight saved, packaging (“accommodation”) flexibility improved and short-term high-power requirements satisfied for vehicle start-up. The desired solution should not require the use of a two-voltage output generator or of a 42/12 V DC/DC converter, depending on the overall electrical load requirement on the 14 V side of the system.
An advantage of the present invention is to use a single battery which allows the energy required for a starting process to be provided in all circumstances. In addition to this, it should be possible to switch to a parallel arrangement when a high battery capacity is required.
Advantageously, the present system includes a 12 V electrical energy storage capability (for the 14V bus) and storage functionality for 36 V devices (for the 42V bus). This avoids the need for a separate 12 V battery, thus saving weight and the required accommodation space. Ideally, the accommodation space requirement for a 36 V device for energy storage would remain the same in comparison with the accommodation space requirement for a 36 V battery in a two-battery arrangement. Because a 36 V battery can be regarded as three series-connected 12 V batteries, the 12 V energy storage function may be provided by the provision of three positive battery poles (e.g., 12 V, 24 V, 36 V) as well as the battery ground.
However, there exist two problems in designing an arrangement as proposed. On the one hand, differences occur in the states of charge of the various cells in the battery, because the battery is charged at a 42 V level and energy is additionally drawn from those cells which provide the functionality for the 12 V part of the electrical energy storage. Secondly, this additional discharging of these cells by the 14 V part of the power network (if no DC/DC converters are available) results in the battery cells carrying out unequal work, so that they have a different aging distribution over time. Since this will shorten the general life expectancy of the battery, a change to this approach is required for the electrical two-voltage power supply system.
In order now to avoid aging of only some of the cells in the 36 V battery, it is necessary in addition to distribute the drawing of electrical energy from the battery over the various cells. In the following text, the 36 V battery is therefore regarded as three separate 12 V parts in a common housing. In this regard, an objective is to ensure that each 12 V part is switched alternately in order to incorporate the 12 V energy storage functionality as part of the electrical 36 V power supply system.
In this context, EP 1 295 757 A2 discloses a power supply system for vehicles having an energy source for supplying electrical power to a large number of electrical load groups, each at different voltage levels. In this case, a large number of load elements form each electrical load group, and a large number of output means are provided in order to deliver the electrical power for each predetermined voltage as a medium voltage from the energy source. In this case, the large number of output means are arranged in the energy source, with the large number of load elements being distributed between a number of blocks in each load group and being connected to each output element in order to ensure that the electrical energy which is being consumed simultaneously by each distributed block is essentially the same. This power supply system is intended to improve the charge equalization in the battery without having to use a special battery indication and monitoring device or a DC/DC converter, and with the desired medium voltage being delivered from the high-voltage battery.
In a modified embodiment of this implementation, the system has an energy source for supplying electrical power to an electrical load group at a low voltage and to an electrical load group at a high voltage. The energy source in this case has output connections at the ends, which are connected to the electrical load group at a high voltage, as well as a large number of central output connections in order to deliver the electrical energy for the electrical load group at the low voltage as a medium voltage from the energy source. In this case, the electrical load group at the low voltage is subdivided into a large number of blocks which are connected in an appropriate manner to each output and to the large number of central output connections, in order to allow the electrical energy which is consumed at the same time by each block to be essentially the same and, furthermore, with one of the blocks which are distributed in the electrical load group at the low voltage being set such that it has more load than the other blocks.
The power supply system which is described in this document is based on the following problem:
A standard 12 V battery (14 V power supply system) has been used for vehicles for many years. However, as a result of the addition of new functions such as electrical devices to assist starting of an engine and to assist its acceleration, such as electrically assisted steering, electrical braking and/or electrical suspension damping systems which are intended to be integrated in the relevant vehicle, the electrical capacity when using 12 V batteries is not sufficient to support these functions. Attempts have therefore been made to provide these new functions at a high voltage and at high electrical power, as well as to efficiently produce electrical energy for them, and thus to use them effectively, with the aim being to connect an energy source in an electrical system for a vehicle to a 36 V battery (to a 42 V power supply system), whose voltage is three times that of the previous systems. However, from the technical and financial points of view, a large amount of effort is required to provide only the 42 V power supply system which is intended to interact with all of the electrical devices in the vehicle, such as the valves for low voltage or the control devices with an electronic control unit. Furthermore, in the case of a solution such as this, the total charge intensity is too low for a 42 V load group. It has therefore been necessary to use a two-voltage system in the form of a 12/42 V voltage system. However, in the known solutions, the superimposition of the charging currents of the 36 V battery part and of the 12 V battery part have resulted in a problem in that the 12 V battery part is not only excessively discharged but that also this discharging of the 12 V battery part makes it necessary to provide an indication and control of the charging currents, for example by means of a costly DC/DC converter. The already described embodiment of the described power supply system is intended to overcome this problem.
Thus, in the case of a 36 V battery, three separate 12 V parts are provided in a common housing, with each 12 V part being switched alternately in order to provide the 12 V energy storage functionality additionally as part of the electrical 36 V energy storage system. In conjunction with an intelligent switching regulator which, for example, may use MOSFET switching elements or relay switching elements, it is now possible to use the 36 V battery as an electrical energy storage device for the 42 V part and for the 14 V part of the electrical system of the vehicle without involving charge differences or aging problems. In this case, a battery indication and monitoring system can be used to ensure that the switching between the three battery parts is controlled as a function of their state of charge.
However, an arrangement such as this is difficult when using electrical two-voltage energy storage systems within an electrical two-voltage system. This is because of the common grounding that is used in present-day vehicles. Specifically, in this case, the 36 V battery ground is connected to the vehicle chassis in this configuration. If the electrical 12 V energy storage functionality were to be provided in this case for the second 12 V part of the 36 V battery (24 V-12 V), the first 12 V part of the 36 V battery would be short-circuited. This necessitates a new cable harness arrangement for each 14 V charging process, because this cannot be directly connected to the vehicle chassis. A solution such as this is undesirable for cost and standardization reasons and, instead of this, it is desirable to improve this solution option.
Against the background of these known power supply systems, the invention is based on the object of improving and further developing the known arrangements while maintaining the previous advantages, in such a way that the disadvantages of the solutions discussed above are avoided, with a particular aim being to avoid the charge differences and aging problems that have been mentioned while the further aim is, in particular, to prevent the short-circuit problem described above. In this case, a further aim is to be able to manufacture the desired arrangement simply and at low cost, and for its life and reliability to comply with the existing requirements.